1. Field of the Invention
This invention relates to an electrodeless arc discharge lamp having a protective coating inside the arc chamber. More particularly, this invention relates to a high intensity electrodeless, metal halide arc discharge lamp with a protective coating of metal silicate on the interior, equatorial portion of the arc chamber.
2. Background of the Disclosure
A new class of high intensity arc discharge (HID) lamps has been developed called electrodeless lamps. Such lamps have a light-transmissive, electrodeless arc chamber or tube generally shaped like a pillbox or slightly flattened sphere and containing a fill which comprises a suitable inert buffer gas and one or more metal halides, including a halide of an alkali metal such as sodium. Radio frequency (RF) energy applied or coupled to the fill via capacitive or inductive coupling generates a light-emitting arc. In operation of such a lamp via inductive coupling, the arc tube or chamber acts as a single-turn secondary coil of a transformer and is surrounded by an RF energy excitation coil which acts as a primary coil. Various embodiments of such lamps are disclosed, for example, in U.S. Pat. Nos. 4,810,938; 4,972,120; 4,959,584 and 5,039,903 all of which are assigned to the assignee of the present invention.
Some of the problems associated with these lamps relate to arc instability and devitrification of the interior surface of the fused quartz arc chamber, particularly at the equatorial portion which is closest to the plasma arc discharge and where the induced electric field is the highest. It is known that the alkali metal present in the fill, such as sodium, forms an alkali metal silicate (i.e., sodium silicate) with the fused quartz arc tube wall, thereby depleting the arc of its sodium content and leaving free halogen behind. This results in accelerated devitrification of the fused quartz, shift in emitted color and promotes arc instability.
Continuing research and development has been directed towards reducing the sodium loss and arc chamber devitrification, while maintaining the relatively high color rendering index (CRI) and lamp efficacy exhibited by these lamps, by applying a protective coating, such as beryllium oxide or silicon, to the interior arc chamber surface as is disclosed in U.S. Pat. Nos. 5,032,762; 5,057,751 and 5,098,326.